The subject of the invention is the manufacture of vitreous metal oxides.
A process for manufacturing vitreous metal oxides according to which a metal alcoholate is subjected to hydrolysis is known. This known process is commonly called the "sol-gel process" (TREATISE ON MATERIALS SCIENCE AND TECHNOLOGY, Vol. 22, 1982, Academic Press. Inc.--S. SAKKA: 37 Gel Method for Making Glass", pages 129 to 167). It generally implies that the hydrolysis is carried out in an acid medium, in order to avoid a precipitation of the metal oxide (ibid. page 132); the production of high acidity in the reaction medium (pH between 1 and 2.5) by the addition of inorganic acids such as hydrochloric acid or nitric acid (U.S. Pat. No. 4,476,156, column 4, line 65 to column 5, line 1) is recommended.
Vitreous lead titanate has been manufactured by means of a process in which titanium isopropoxide was first reacted with lead acetate dissolved in methoxyethanol, the resulting alcoholate was dissolved in methoxyethanol and it was subjected therein to hydrolysis using a solution of methoxyethanol and water containing nitric acid (Ultrastruct. Process. Ceram. Glasses, Compos. (Proc. Int. Conf. (1984), pages 152-160)). In this known process, the solution of methoxyethanol and water employed contained 0.002 mole HNO.sub.3 /mole of water; the strict necessity to employ more than 0.0005 mole HNO.sub.3 /mol of water is stressed (page 155).
The presence of strong inorganic acids in the reaction medium is a source of disadvantages. It is especially a source of impurities in the metal oxide glass, which have an adverse effect on its transparency. In the case where the vitreous metal oxide is formed in situ on a substrate, the inorganic acid is likely to cause corrosion of the substrate. Moreover, the presence of inorganic acid implies that the equipment used for the hydrolysis reaction is made of expensive materials, which are resistant to corrosion by acids.